The Cell Cycle Flashcards
1
Q
A
2
Q
Define Cell Cycle
A
The regulated sequence of events that occurs between one cell division and the next
3
Q
What does the length of the cell cycle depend on? How long is it usually in human cells? What about bacteria?
A
- Length depends on environmental conditions, the cell type and the organism
- Lasts 20-24 hours in humans
- About 20 mins in most bacteria
4
Q
What triggers the movement from one phase to another?
A
Chemical signals called cyclins
5
Q
What happens in the G1 phase of interphase?
A
- The cell grows in size
- Will receive a signal to divide again at some point
- Once this happens, it will make the RNA, enzymes and other proteins required for growth and DNA synthesis
- Cell activates its centromeres to prepare itself for division
6
Q
What happens during the S phase of interphase?
A
- The DNA is replicated by semi-conservative replication
- So that each chromosome consists of 2 identical sister chromatids
- NB Cell growth stops during this phase
7
Q
What happens during G2 of interphase?
A
- Cell continues to grow
- Newly synthesised DNA is checked and errors are repaired
- All organelles replicate
- Centrioles replicate (1 —> 2)
- Tubulin is produced - to make microtubules for spindle fibres
8
Q
What proportion of the cell cycle is Interphase?
A
75-95% - normal cellular functions continue during this period
9
Q
Are there any eukaryotic cells that don’t go through the cell cycle?
A
Yes, not all euk. cells retain the ability to divide
10
Q
What happens during prophase?
A
- Nuclear envelope and nucleolus disintegrate into small vesicles
- Chromosomes condense and become visible under the microscope
- Centrioles migrate to opposite ends of the cell and spindle fibres begin to emerge from them
11
Q
What happens during metaphase?
A
- Chromosomes align along the equator of the cell (metaphase plate)
- Centrioles release spindle fibres, which attach to the centromeres of the chromosomes
12
Q
What happens during anaphase?
A
- Spindle fibres contract and shorten, pulling sister chromatids to opposite poles
- Centromeres split in two
13
Q
What happens during telophase?
A
- New nuclear membranes begin to reform around both nuclei
- Cytoplasm begins to separate
- Chromosomes begin to uncoil (decondense) and you can no longer see them under a microscope
14
Q
How is mitosis used in multicellular organisms?
A
- Growth
- Replacement of cells and repair of tissues
- Asexual reproduction
15
Q
What happens in cytokinesis in animal cells?
A
- Cytoplasm splits in 2 - forming 2 genetically identical cells
- A cleavage furrow forms around a contractile ring (generates constricting force to separate the cell into 2 - plasma membrane can then form there)
16
Q
What happens in cytokinesis in plant cells?
A
- A cell plate forms on the equator of the parent cell and expands towards the cell wall
- Where a cell wall will form once it has reached the cell wall
17
Q
How to identify prophase from a photomicrograph?
A
- Chromosomes are condensed and visible and nuclear envelope is disintegrating
18
Q
How to recognise metaphase on a photomicrograph?
A
- Chromosomes are lined up along the equator (middle) of the cell
19
Q
How to recognise anaphase on a photomicrograph?
A
- Chromosomes are moving away from the middle of the cell, towards opposite poles
20
Q
How to recognise telophase on a photomicrograph?
A
Chromosomes have arrived at opposite poles and begin to decompose. Nuclear envelope is reforming.
21
Q
How to recognise cytokinesis (not mitosis)
A
- Cleavage furrow or cell plate on the metaphase plate
22
Q
How to calculate mitotic index
A
Number of cells w. visible chromosomes / number of cells in field of view
23
Q
Outline the process of binary fission [4]
A
- The single, circular DNA molecule undergoes DNA replication (semi-conservatively)
- Any plasmids present undergo DNA replication
- The cell elongates
- The parent cell divides into two cells, with the cytoplasm roughly halved between the two daughter cells
- The two daughter cells each contain a single copy of the circular DNA molecule and a variable number of plasmids
24
Q
When are the checkpoints in the eukaryotic cell cycle?
A
near the end of G1, at the G2/M transition and during metaphase
25
What do they each check?
G1 - checks all the proteins, RNA and enzymes required for growth and DNA replication for damage
G2 - DNA is checked for errors, also all the newly replicated organelles
Metaphase checkpoint - checks that all the chromosomes are attached to 2 spindle fibres before mitosis can continue
26
What happens if a cell fails a checkpoint?
- The cell will attempt to fix it and cell cycle goes on as normal if this is successful
- If it isn’t, the cell undergoes apoptosis (programmed cell death)
27
What happens when chromosomes condense?
Euchromatin —> Heterochromatin
28
What happens in binary fission if a daughter cell does not receive the single circular DNA molecule or at least one copy of a plasmid?
It dies
29
Name 2 causes of uncontrolled cell division
- Carcinogens (eg UV, Tobacco smoke)
- Oncoviruses (viruses that cause cancer
30
What causes the generation of cancerous cells?
A mutation in the genes in the DNA that control mitosis and the cell cycle - eg the p53 gene
31
Why can’t mutations that result in the generation of cancerous cells be easily cured by the body?
- Although cancer cells have different antigens on their CSM
- These mutations aren’t destroyed by the body’s immune system or at the G2 checkpoint and don’t result in apoptosis
- Therefore the mutation is passed on to all the cell’s descendants
- And this happens fast because the cell division is uncontrolled
- Therefore cancerous cells divide repeatedly and uncontrollably to form a tumour
32
What are oncogenes?
- Mutated genes that cause cancer
33
Why are tumours dangerous?
- Tumours develop rapidly and exponentially expand in size as the cell cycle is much shorter
- Therefore a typical tumour contains around 1 billion cancerous cells by the time it is detected
- These cells consume nutrients and energy needed by the body (lots of ATP is required) and so the body’s chemistry changes
- This gives the symptoms of cancer ie weight loss, sickness, fatigue
34
Why is it important that the cell cycle is strictly regulated?
- Because otherwise tumours form
35
Define benign tumour and give examples
A tumour which:
- Grows slowly
- Doesn’t spread from its original site
- Doesn’t cause cancer so isnt life threatening
eg warts/ uterine fibroids
36
Define malignant tumour
A tumour which causes cancer:
- Spreads through the body
- Grows rapidly and is less compact
- Invades and destroys other tissues
- Also interferes with the normal functioning of the organ/tissue in which it has started to grow - for example by blocking the lungs/intestines/blood vessels
37
How does a malignant tumour spread through the body?
By a process called metastasis:
- Their cells break off the tumour
- And travel through the blood/lymphatic system to other parts of the body
- To cause secondary growths in different parts of the body
38
Why is metastasis dangerous?
- As it is very hard to detect, locate and remove secondary growths in the body
- This is why cancers cause death
39
How do chemotherapy drugs work?
- They control the rate of mitosis
- Kills dividing cells by blocking a part of the cell cycle
40
How does Methotrexate treat cancer?
- It is a chemotherapy drug
- Which inhibits the S phase of interphase
- By inhibiting DNA replication
41
How do Vincristine and Taxol treat cancer?
- Chemotherapy drug
- Which inhibits Metaphase
- By preventing the formation of spindle fibres from the centrioles
42
Why is chemotherapy problematic?
- It kills healthy cells as well
- Especially those that have a short cell cycle eg hair cells (this is why hair loss happens)
43
How is radiotherapy used to treat cancer?
- A huge shot of high-energy radiation causes mutations in the DNA
- So many mutations are caused that it results in apoptosis at the G2 checkpoint
- This shrinks tumours or completely eliminates them
44
Why is it so hard to treat cancer?
- There can be many other reasons why the control of cell division deteriorates other than a mutated p53 gene.
- Therefore there is no single effective treatment for individuals suffering from cancer.
